Plant Responses, Hormones, and Development Stages

Posted on Mar 18, 2025 in Biology

Item 2: Plant Responses, Hormones, and Development

The Relation (Stimulus-Response)

Relation is the capacity to formulate a response to a stimulus, i.e., a change that can be both internal and external to the plant. This change is detected by receptors located in the epidermis of the plant. The responses are regulated by phytohormones. Receptors are classified according to the stimuli they detect:

  • Photoreceptors: Detect changes in light intensity.
  • Chemoreceptors: Detect chemical changes.
  • Thermoreceptors: Detect changes in temperature.
  • Mechanoreceptors: Detect changes in pressure.

Plant hormones are produced in poorly differentiated tissues, such as embryonic and meristematic tissues. Phytohormones traverse the cell wall to reach the conducting vessels, and from there, they reach the so-called target cells where they trigger their effects.

B. Movements

Movements occur as a result of hormonal effects:

b1) Tropism

Tropism is a continuous movement of the plant, or one of its organs, toward the stimulus. If it moves *towards* the stimulus, it is said to be positive, and if it moves *away*, it is negative. Examples of tropism include:

  • Phototropism
  • Geotropism
  • Thigmotropism
  • Chemotropism

b2) Nastic Movements (Nasty)

Nastic movement is a non-permanent movement that occurs only in the presence of the stimulus. It is a reversible movement. Examples include photonasty and seismonasty.

C. Stages of Plant Development

c1) Seed Germination

The seed protects the embryo and ensures its dispersion. If environmental conditions are favorable, germination occurs, i.e., the growth and development of the embryo.

Seeds are produced in autumn and go through a dormant phase (latency) until spring, which is detected by three hormones:

  1. Abscisic acid: Prevents the growth of the embryo but favors the storage of reserve substances.
  2. Gibberellins: Stimulate germination and the use of reserve substances.
  3. Auxins: Intervene in the downward growth of the root and the upward growth of the stem.

c2) Vegetative Growth

The plant begins growth in length and thickness when it can absorb water and mineral salts and carry out photosynthesis. The following hormones are involved in this stage:

  1. Auxins: Induce apical stem growth and branching of the root.
  2. Gibberellins: Induce growth in stem length.
  3. Cytokinins: Develop the photosynthetic apparatus in leaves and the presence of chlorophyll.

c3) Flowering

Flowering is the beginning of the reproductive phase. It depends on the photoperiod, i.e., the ratio of hours between day and night. The following are involved in flowering:

  1. Phytochrome: A pigment.
  2. Florigen: A hormone-like substance.

Flowering is also induced by cytokinins.

c4) Formation of the Seed and Fruit

After pollination and fertilization, seed and fruit development are governed by:

  • Cytokinins: Involved in seed development.
  • Auxins or gibberellins: Stimulate the ovary to form the fruit.
  • Ethylene: Involved in fruit ripening, which breaks down chlorophyll and transforms starch reserves into sugars.

c5) Senescence

With the fruit ripe, its fall (abscission) occurs, the flowers wilt, and the leaves fall. The following are involved:

  1. Abscisic acid and ethylene: Cause the formation of a series of enzymes that digest the cell wall of the cells of old tissues as they fall.